This invention relates to semiconductor devices and a process for the fabrication of the same, and particularly to formation of insulating layers in accordance with optically assisted anodizing process.
Recently, high integration in semiconductor devices has been developed with remarkable rapidity so that the device has progressed from a small scale integrated circuit (SSI) to a very large scale integrated circuit (VLSI).
Such high integration of semiconductor integrated circuits is realized by miniaturizing elements. Thus fine and highly precise pattern formation technique has been desired earnestly.
For example, sandwich type photoelectric converting devices in which amorphous silicon layer is utilized as a photoelectric conversion layer have attracted attention lately in order to use them as contact type image sensors. This type of photoelectric converting device has generally a construction as shown in FIG. 1 wherein a plurality of metal electrodes 2 are juxtaposed on an insulating substrate 1, and thin hydrogenated amorphous silicon film as a photoelectric conversion layer 4 is sandwiched between these metal electrodes and a transparent electrode 3. In such construction of the photoelectric converters, it has been considered that the insulation between adjacent electrodes is sufficient, because a distance D between the adjacent electrodes is generally ten or more times larger than the thickness d of the thin hydrogenated amorphous silicon film. In other words, because the distance between the adjacent electrodes is 10 or more times the thickness of the thin hydrogenated amorphous silicon film, the insulation across the adjacent electrodes is sufficiently assured for the practical use of the photoelectric converter. However, such problems as described hereinbelow appear more frequently as the density of the elements becomes higher in order to improve the resolution of the reproduced image. In other words, as the density of elements becomes higher in order to improve the resolution in the reproduced image, such problems as described hereinbelow arise. Namely, in making the density of the elements higher, since area of each electrode must be maintained, distance between the electrodes, i.e., the distance D, should be smaller. Thus, the distance D of the electrodes becomes close to the thickness d of the hydrogenated amorphous silicon film, i.e., distance between the metal electrode 2 and the transparent electrode 3. As a result, the insulation between the metal electrodes becomes insufficient. Thus, there will arise a problem in which a current is likely to flow across adjacent electrodes through the photoelectric conversion layer, resulting in lowering the performance of the photoelectric converters as a sensor.
To solve the above problem, there has been proposed, as shown in FIG. 2, a construction wherein an auxiliary electrode 5 having a width l is disposed between the metal electrodes each having a width L (l&lt;&lt;L) and juxtaposed on an insulating substrate with a distance D.sub.1. At the same time, a bias having the same polarity with that of the transparent electrode is applied to the auxiliary electrodes.
According to the above construction, a current will not flow across adjacent electrodes through the photoelectric conversion layer. However, there is a limit to the shortening of the distance D.sub.1 between the metal electrodes due to the existence of the auxiliary electrodes. In addition, the cost for the production becomes expensive because the number of fabricating processes, including the formation process for auxiliary electrodes, is increased.
Furthermore, in semiconductor devices having an amorphous semiconductor layer other than a contact type image sensor, when an amorphous semiconductor layer is subject to a patterning operation in accordance with a photolithographic method or the like, deterioration due to contamination by impurities and the like is likely to occur. Besides, the amorphous semiconductor layer is weak in the context of high-temperature processing. Hence it is very difficult to form an oxide layer after formation of the amorphous semiconductor layer.